Sexual activity consists of a desire phase followed by an erectile phase when successful intercourse is possible. This results in build up of some hormones and enzymes, which causes ejaculation and orgasm mechanism to trigger off almost simultaneously. The blood flows out of the penis which then becomes flaccid so much so that another erection is possible only after some time. This period is technically called the refractory period, the duration of which increases with advancing age.

As age advances the number of sexual encounters decrease. So if at 25 years of age the number of encounters is ‘X’ ,by 50 years the same is reduced to x/2. However, the desire does not reduce in the same proportion. Herein lies the frustration – the so called middle age crisis. It all starts from the bedroom. That is why one has to work seriously at creating the right atmosphere, the right ambience with passage of time. In actual fact, reverse is seen. With passage of time, couples tend to take each other for granted so that intimacy and sex loses its charm. In fact, sex begins from breakfast – a peck on the  cheek or a light hearted comment as you go to the office goes a long way in not only having a great  day but also great sex which in turn results in deep and healthy sleep. A little bit of effort can bring so much change in your life! Don’t believe me. Try it for yourself and see the change.   

You can read more on the paradoxes of sexual desire here

https://link.springer.com/chapter/10.1007%2F978-1-4899-0852-0_5